In known medical image processing systems, a user is able to create a mesh that defines an edge of a volume of interest (e.g., a heart chamber) in a 3D image. Upon defining a mesh, the user is able to place annotation objects to mark places of interest. These annotation objects, which are themselves 3D mesh objects, can be placed either on the mesh or inside of the chamber hidden by the mesh outlining the chamber. Obscuring annotation objects in a medical or other type of image is likely to cause a problem and render the user unaware of potentially important objects. Known systems may allow opacity of the chamber mesh in the image to be reduced to promote visibility of the annotation. However reducing opacity reduces contrast level and visibility of features in the image. A system according to invention principles addresses these deficiencies and related problems.